This invention relates to a four wheel drive system for a vehicle, wherein power circulation is prevented, thereby eliminating excessive loads, worn tires and deteriorated fuel consumption and the like due to power circulation.
Recently, as the drive systems used in vehicles such as motor vehicles, four wheel drive systems have become popular. Heretofore, there have been two types of such four wheel drive systems. The first is one having a two wheel - four wheel switching control clutch, wherein front wheels and rear wheels are selectively driven and connected to each other. The second is one having a center differential gear and a differential control clutch for restricting (up to the degree of prohibiting) the differential action of the center differential gear, whereby the front wheels and the rear wheels can be directly or almost directly connected to each other. These are disclosed in bulletins of Japanese Patent Kokai (Laid-Open) Nos. 72420/1980, 53520/1983 and 101829/1983.
When the above-described release and restriction of differential action between front wheels and rear wheels are controlled in response to an outside signal, an arrangement may be adopted in which the differential action is restricted throughout the running time of the vehicle or during most of the running time, and the restriction of differential action is suitably released (or the degree of the restriction changed) in accordance with the running conditions of the vehicle. Or, an arrangement may be adopted in which the differential action between the front wheels and the rear wheels is permitted during normal operation, and the restriction of differential action is applied when necessary.
In general, when the difference in rotary speed between front wheels and rear wheels is increased, it is recognized that either the front wheels or the rear wheels are slipping relative to the road surface, whereupon the differential action is restricted.
However, conventional four wheel drive systems have not taken into account differences in effective radii of tires caused by changed in weight and load acting on front wheels or rear wheels, change in pneumatic pressures within the tires, changing wear in the tires, use of a temper tire, attachment of chains to the tires, and the like. Accordingly, if the restriction of differential action is performed in the case when the such differences exist in the effective radii of the tires, power circulation occurs, whereby excessive load, abrasion of tires, deterioration in fuel consumption and the like may be induced.